Attention has been focused on a technique for forming a transistor using a semiconductor film that is formed on a substrate having an insulating surface. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. A silicon film is known as a semiconductor film applicable to a transistor.
As a silicon film used as a semiconductor film of a transistor, either an amorphous silicon film or a polycrystalline silicon film is used depending on the purpose. For example, in the case of a transistor included in a large display device, it is preferable to use an amorphous silicon film, which can be formed using the established technique for forming a film on a large substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferable to use a polycrystalline silicon film, which can be used to form a transistor having a high field-effect mobility. As a method for forming a polycrystalline silicon film, high-temperature heat treatment or laser light treatment that is performed on an amorphous silicon film has been known.
In recent years, a transistor that includes an oxide semiconductor film (typified by an oxide semiconductor film containing indium, gallium, and zinc) has attracted attention.
An oxide semiconductor film can be formed by a sputtering method or the like, and thus can be used for a transistor included in a large display device. Further, a transistor including an oxide semiconductor film has a high field-effect mobility; therefore, the use of the transistor allows fabrication of a high-performance display device where driver circuits are formed over the same substrate. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
As a method for providing a transistor including an oxide semiconductor film with stable electrical characteristics, a technique where an insulating film in contact with an oxide semiconductor film is doped with oxygen is disclosed (see Patent Document 1). The technique disclosed in Patent Document 1 enables oxygen vacancies in an oxide semiconductor film to be reduced. As a result, variation in electrical characteristics of a transistor including an oxide semiconductor film can be reduced and reliability can be improved.
A transistor including an oxide semiconductor film is known to have an extremely low leakage current in an off state. For example, a low-power semiconductor device and the like utilizing the low leakage current of a transistor including an oxide semiconductor film and the high on-state current of a silicon transistor are disclosed (see Patent Document 2). In addition, for example, a CPU with low-power consumption utilizing the low leakage current of a transistor including an oxide semiconductor film is disclosed (see Patent Document 3).